By Michael Slezak It underpins the whole theory of quantum mechanics, but does it exist? For nearly a century physicists have argued about whether the wave function is a real part of the world or just a mathematical tool. Now, the first experiment in years to draw a line in the quantum sand suggests we should take it seriously. The wave function helps predict the results of quantum experiments with incredible accuracy. But it describes a world where particles have fuzzy properties – for example, existing in two places at the same time. Erwin Schrödinger argued in 1935 that treating the wave function as a real thing leads to the perplexing situation where a cat in a box can be both dead and alive, until someone opens the box and observes it. Those who want an objective description of the world – one that doesn’t depend on how you’re looking at it – have two options. They can accept that the wave function is real and that the cat is both dead and alive. Or they can argue that the wave function is just a mathematical tool, which represents our lack of knowledge about the status of the poor cat, sometimes called the “epistemic interpretation”. This was the interpretation favoured by Albert Einstein, who allegedly asked, “Do you really believe the moon exists only when you look at it?” The trouble is, very few experiments have been performed that can rule versions of quantum mechanics in or out. Previous work that claimed to propose a way to test whether the wave function is real made a splash in the physics community, but turned out to be based on improper assumptions, and no one ever ran the experiment. Now, Eric Cavalcanti at the University of Sydney and Alessandro Fedrizzi at the University of Queensland, both in Australia, and their colleagues have made a measurement of the reality of the quantum wave function. Their results rule out a large class of interpretations of quantum mechanics and suggest that if there is any objective description of the world, the famous wave function is part of it: Schrödinger’s cat actually is both dead and alive. “In my opinion, this is the first experiment to place significant bounds on the viability of an epistemic interpretation of the quantum state,” says Matthew Leifer at the Perimeter Institute in Waterloo, Canada. The experiment relies on the quantum properties of something that could be in one of two states, as long as the states are not complete opposites of each other: like a photon that is polarised vertically or on a diagonal, but not horizontally. If the wave function is real, then a single experiment should not be able to determine its polarisation – it can have both until you take more measurements. Alternatively, if the wave function is not real, then there is no fuzziness and the photon is in a single polarisation state all along. The researchers published a mathematical proof last year showing that, in this case, each measurement you make reveals some information about the polarisation. In a complicated setup that involved pairs of photons and hundreds of very accurate measurements, the team showed that the wave function must be real: not enough information could be gained about the polarisation of the photons to imply they were in particular states before measurement. There are a few ways to save the epistemic view, the team says, but they invite other exotic interpretations. Killing the wave function could mean leaving open the door to many interacting worlds and retrocausality – the idea that things that happen in the future can influence the past. The results leave some wiggle room, though, because they didn’t completely rule out the possibility of some underlying non-fuzzy reality. There may still be a way to distinguish quantum states from each other that their experiment didn’t capture. But Howard Wiseman from Griffith University in Brisbane, Australia, says that shouldn’t weaken the results. “It’s saying there’s definitely some reality to the wave function,” he says. “You have to admit that to some extent there’s some reality to the wave function, so if you’ve gone that far, why don’t you just go the whole way?” Journal reference: Nature Physics, DOI: 10.1038/nphys3233 More on these topics: